A conventional shift range control apparatus switches over a shift range by controlling a motor of a shift range switchover device in response to a shift range switchover request from a driver. For example, in a shift range switchover device disclosed in JP 2004-308752A (US 2008/0168853A1, US 2008/0210033A1, US 2006/0207373A1), a switched reluctance motor (SR motor) is used as a motor for a drive power source.
The shift range control apparatus according to JP 2004-308752A performs a reference position learning control for learning a reference position of a motor by rotating the motor until a detent plate, which is a driven target of the motor, stops at a limit position in a movable range before starting a normal driving control of the motor. By learning the reference position, the limit position and the reference position of the motor are matched so that the motor may be rotated to a target rotation position by the normal driving control.
When the reference position learning control is performed, the detent plate or a roller of a detent spring hits a wall, which corresponds to the limit position in the movable range. Since inertia torque of the motor at this time is a load to a wall in respect to designing strength of the motor, it is desired to control a rotation speed of the motor to a low rotation speed so that the detent plate hits the wall at low speed. In the shift range control apparatus described above, the rotation speed of the motor is lowered by shortening an ON period of a current supply command pulse applied to the motor in performing the reference position learning control. In case that an SR motor is used as a motor for the driving power source of the shift range switchover device, torque of the motor will not increase even when the motor stops rotation after hitting the wall in performing the reference position learning control.
In place of the SR motor for the shift range switchover device, a DC brushless motor, which has a good response characteristic, is proposed to be used. In case that the DC brushless motor is used as the motor for the shift range switchover device, torque of the motor will increase when the motor stops rotation after hitting the wall in performing the reference position learning control. For this reason, it is required to limit the torque of the motor, that is, a driving current supplied to the motor. In case that the driving current is not detected because of a detection circuit failure or the like, it is not possible to limit the torque of the motor appropriately.